Substituent effects on the TiO2 photosensitized oxidation reaction of benzyl thioethers and thiols in deaerated acetonitrile

Abstract

Abstract The TiO 2 photosensitized oxidation of benzyl methyl sulfides (X-C 6 H 4 CH 2 SCH 3 ) and benzyl thiols (X-C 6 H 4 CH 2 SH) has been investigated in Ar-saturated CH 3 CN. Steady-state irradiation produced benzaldehydes or dibenzylsulfides as oxidation products with sulfides and thiols, respectively. The results obtained through kinetic competitive experiments, aimed to evaluate the ring substituent effect on the reactivity, suggested the involvement of radical cation intermediates, formed by the favorable electron transfer from the substrate to the TiO 2 photogenerated hole, which reasonably deprotonate to give the final products. This process was poorly affected by the adsorption of the substrate at the TiO 2 surface, as demonstrated by similar results, both in terms of products and reactivity, collected for the homogeneous photooxidation of the same substrates sensitized by N -methoxyphenanthridinium hexafluorophosphate (MeOP + PF 6 − ). This behavior is likely due to the low hydrogen-bond acceptor ability of divalent sulfur systems. Density functional theory calculations pointed out that the most stable conformations of X-C 6 H 4 CH 2 SH + are characterized by having the C S bond almost collinear with the π system of the aromatic ring and by a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom.